The odor of love.
Sex of the salmon
Face-on view of Helix aspersa, a garden snail. Both pairs of tentacles are sensitive to chemicals; the upper set also detects light.
Courtesy Cathy Levesque
POSTED 12 JULY 2001
What you are about to read is true. Weird, but true. It's news from the battlefront between the sexes -- molluscan style. (Molluscs include snails and slugs.)
The story concerns the garden snail. Helix aspersa is found in California and South America. The snail is native to Europe, where it may have picked up its curious courtship and mating rituals.
Consider: Like many air-breathing snails, they are hermaphrodites. Hermaphrodites can take male or female roles in mating and reproduction, as circumstances dictate. The garden snail fulfills both sexual roles simultaneously.
It gets weirder.
These snails punctuate their mating ritual by puncturing their partners with a calcified "love dart." You could call it a shot in the dark, because the animals can detect light and dark but can't see to aim the tiny "Cupid's arrow."
Love darts are strangely common: 17 of the 65 families of terrestrial snails use them.
As if that isn't bizarre enough, this species engages in mutual copulation -- they mate with each other.
Early courtship in Helix aspersa. Note swollen genitals in lefthand snail.
Courtesy Ronald Chase
With snails, as with zebras, fireflies and humans (and horseshoe crabs for all we know), mating begins with a courting ritual.
Ronald Chase, a professor of biology at McGill University, makes a living by studying the sexual habits of snails. He says garden snails court from 15 minutes to six hours by circling each other, touching with tentacles, and biting on the lip and genitals. Just before mating, hydraulic pressure builds up in the blood sinus surrounding the organ housing the dart, and when the second animal touches the darter's genitals, it fires that dart.
After snail number two responds by firing its own dart, the snails simultaneously mate. She/he does unto him/her while he/she does unto her/him. And for every mating except their first, both Helix aspersa fire the dart. (The tiny harpoon forms after the first mating.)
Two copulating animals. The dart on the right is quite obvious. Ouch! Can you see the second dart?
Courtesy Ronald Chase
We hoped you would wonder about that. In a new study, Chase and colleague David Rogers showed that a chemical signal thought to be carried on the dart prevents the mate from digesting the usual amount of sperm.
The female reproductive tract in many animals, Chase says, is hostile to sperm, because better sperm yield better young. "Basically, she's making it tough so only the best sperm are going to make it," Chase says. "Those are the sperm she wants." (The sperm-digesting snail also gets a protein-rich snack in the process.)
"Tough" doesn't begin to describe the tribulations of sperm inside Helix aspersa. If the "female" snail does not get the signal from the dart, she digests 99.98 percent of the sperm. That can make it hard for a guy to have kids, especially since the garden snail is rather, shall we call it promiscuous, and likely to be fertilized by the guy/gal on the next barstool.
But Chase and Rogers found that snails that had been darted stored 116 percent more sperm than undarted snails. That confers a major reproductive advantage.
The little game of darts is a new plot twist in the ancient saga of competition among guys to make young, specifically the chapter that biologists call sperm competition.
"The battleground of sperm competition is well documented in many birds and insects," Chase says. Sperm exist for one purpose - to fertilize eggs -- and males and sperm use behavioral and chemical tactics to reach that destiny:
- Some males guard their mates from other males after mating.
- Others leave a blocking device in the female's vagina preventing entry of another male's reproductive apparatus.
- A chemical in fruit-fly semen reduces the female's motivation to have sex again.
- Semen may chemically attack sperm left from previous matings, which presumably came from competing males.
So the supposition that slinging darts could help a snail sire super surpluses of offspring by sparing sperm a savage assault is starting to seem sensible.
But is this whole concept of hermaphroditism sticking in your craw? Any fan of horse breeding or soap operas knows that the sexes are designed to mix genes. If hermaphroditism is so useful, how come they're as rare on the Young and the Restless as infants themselves?
Time for a good word for the evolutionary benefits of switch-hitting. Plants routinely play both roles, says Chase. Among animals, it's less common, although 40 percent of 5,600 genera of molluscs are hermaphrodites -- including all land-dwelling molluscs.
Snails, you'll recall, move along at a snail's pace, and slugs are a bit sluggish. And if you were a slow-moving member of a rare snail species, you might never meet a mate (remember, biologically, for a guy, being marooned with a bunch of he-men equals a life without offspring -- literally a fate worse than death). If you were colonizing new terrain, it would certainly help if you could mate with any adult of your species, not just half of them.
The ultimate ability in these situations would be one that some hermaphrodites actually have -- to fertilize yourself. That might seem rather devoid of pleasure, but it's all in a day's work in the sexual life of snails.
-- David Tenenbaum